Blender and three.js recent versions December 1, 2018
I'm not sure why the three.js exporter would have disappeared from the specific folder in your screenshot (did you download it from somewhere particular? was it already on your computer?) but the three.js (JSON) Blender exporter has been removed from recent versions of the three.js library.

Instead, I would suggest one of the following:


Load your FBX (.fbx) file directly with THREE.FBXLoader.
Export to glTF (.gltf or .glb) and load with THREE.GLTFLoader. 


A glTF exporter is included in Blender 2.80+. For older versions of Blender, you can download the importer/exporter here: https://github.com/KhronosGroup/glTF-Blender-IO.

Per three.js documentation, using glTF is recommended where possible.